The efficiency of conventional power plants has been markedly improved with the integration of the combustion turbine and a variety of heat recovery techniques. To improve energy efficiency and enhance the environmental acceptability of fossil fuels, it is advantageous to include a gasification stage in combined cycle power plants. In these systems a carbonaceous fuel such as coal is converted to syngas, a gaseous mixture formed during a high temperature partial oxidation. This combination of features is commonly referred to as an Integrated Gasification Combined Cycle, or IGCC.
In an IGCC the syngas is fed to a combustion turbine from which exhaust heat is applied to generate steam for a subsequent power stage, and/or to heat incoming materials associated with the combustion cycle. Components of syngas vary considerably depending on the fuel source and reaction conditions. For coal gasification, typical constituents of syngas include CO2, CO, H2 and CH4. Often syngas will include sulfides and nitrous components. The latter are normally removed from the mixture prior to combustion in order to provide an environmentally cleaner exhaust gas from the combustion turbine.
The IGCC is coming into greater use in power production because the overall efficiencies are attractive and the technology presents greater opportunities to deploy coal, an abundant resource, in an economical and clean manner. The efficiency advantage of burning gasified coal in power plants stems in part from the combined cycle, wherein hot gases leaving the combustion turbine are used to raise steam temperature in a conventional Rankine cycle. With a typical gasification efficiency of about 80 percent, and a combined cycle efficiency (combustion and steam turbine) of about 58 percent, it is possible to achieve an overall plant efficiency of 46.8 percent. By way of comparison, the overall efficiency of a typical steam turbine power plant is less than 40 percent although newer ultrasupercritical cycle designs may approach efficiencies up to 45 percent.
More generally, the cold gas efficiency should be at least 78 percent to render the IGCC commercially attractive. The efficiency of the coal gasification process is dependent in part on the gasification temperature which, in turn, is a function of the reactivity of the coal species. It is desirable to react the coal at as low a temperature as possible, as this will maximize the heating value in the syngas relative to the feedstock. However, due to the equilibrium dynamics of the conversion process, reaction temperatures range from about 1400° C. to about 1500° C. (2550° F. to 2730° F.) for various coal species. As a result, gasification efficiencies above 80 percent have been difficult to achieve in large scale commercial operations. Given these constraints, other means of improving the efficiency of gasification are sought, as even small improvements in plant efficiency have large impacts on the cost and viability of energy production from carbonaceous solid fuel sources.